What's the deal with the mpfr versioning? libmpfr4 vs. libmpfr6

[Anders Andersson <pipatron@gmail.com>]

I just noticed that there are two packages for libmpfr:

https://packages.debian.org/sid/libmpfr4

https://packages.debian.org/sid/libmpfr6


Funny thing is, this is what the versioning says on those pages:
Package: libmpfr4 (3.1.6-1)
Package: libmpfr6 (4.0.1-1)

...ok, that's strange. Even weirder, they are both built from the same
sources: mpfr-4.0.1-1.

I feel like I'm missing something. For example, what does the
"3.1.6-1" mean in libmpfr4? I assumed it was the underlying version
but it can't be if it's built from the 4.0.1 sources?

And why libmpfr6 when it's actually version 4 (there's no version 6 of mpfr).